Americans looking to buy seaside property would do well to study the first ever nationwide map showing how and where natural habitats like reefs and vegetation best protect coastal residents from rising seas and catastrophic storms like last year's Hurricane Sandy. (See "Hurricane Sandy Pictures: Floods, Fire, Snow in the Aftermath.")

Shoreline engineering like seawalls can be effective but also expensive, environmentally undesirable, and a detriment to tourism and seaside recreation. But conserving and restoring nature's own coastal habitats can also help save lives. Now Stanford University's Katie Arkema and colleagues have provided a national map of where natural habitats do most reduce risk to people and property—and where they may need help.

Some two-thirds of the U.S. coast is currently protected by one or more of these helpful habitats, according to the study.

Mapping a "Hazard Index"

Arkema and colleagues mapped coastal habitats to create a "hazard index" that evaluated every square kilometer of the U.S. coastline under five different scenarios of sea-level rise. The team then added the coast's human geography, illustrating where people and property stood in harm's way with demographic data from the U.S. Census Bureau and property values developed, in part, from a relationship with the online real-estate service Zillow. (See coastline pictures.)

"It's not just about whether habitats are capable of providing coastal protection," said Arkema, a marine ecologist with Stanford Woods Institute for the Environment.

"It's also where they matter for people and for property," she said. "We really wanted to figure out where habitats are reducing exposure and also where those locations overlap with coastal property values and human populations that need to be protected."

When the team modeled U.S. coastlines with these natural protections removed the results were dire, suggesting that the loss of such habitats would double the stretches of coastline now highly exposed to floods and storms and expose an additional 1.4 million Americans to such threats.

"That really surprised me," Arkema said. "It does make sense. We know for example that there are a lot of people in Florida, and that Florida gets hit with a lot of hurricanes, but when I saw that the totals actually doubled I was really surprised."

Peter Kareiva, chief scientist for The Nature Conservancy, said the study was the first of its type to be really proactive.

"With other studies a disaster comes along, say a tsunami, and afterward people collect information and say, 'Here where they left the mangroves intact, people didn't seem to suffer as much.' That's good science but it's after the fact.

"This study takes us in a direction of saying let's be proactive," he continued. "Let's not wait for a storm to happen. Where does natural habitat offer some natural risk reduction before the storm happens?"

Which States Need Mother Nature the Most?

The study shows that some areas of the U.S. receive much more natural protection than others. Under all scenarios of sea-level rise and storms the East Coast and Gulf Coasts proved generally more vulnerable than the West Coast.

"The East Coast and Gulf Coast, in general, are lower-lying and sea level rise will impact those coasts more," Arkema said.

"Many of the shorelines aren't as hard, there are more muddy or sandy areas. Of course California has many sandy beaches but much of the West Coast tends to be higher elevation, harder shoreline—think of places like Big Sur for example." (Related: "Sea Levels Rising Fast on U.S. East Coast.")

The results also break down state by state. Coastal habitats currently protect the most coastline in Florida, North Carolina, and Alaska. But when the population demographics are taken into account, they protect the most people and property in New York, California, and Florida. (Related: "New York's Sea-Level Plan: Will it Play in Miami?")

Risks and consequences vary dramatically right down to the local level. In Jefferson County, Florida, the value of property protected by coastal habitat was $0 because there is no residential development within 0.6 mile (a kilometer) of the shoreline. (Inland homes do benefit from local wetlands and forests but weren't included in the study figures, Arkema said.)

But in Suffolk and Kings Counties, New York, the figure tops $20 billion. That surprised Arkema, and not because New York property values were so high. "Many people would know those areas are highly developed," she explained, "but it's surprising that they are still surrounded by ecosystems, wetlands, and forests that are relatively intact."

And dollars and cents don't always tell the whole story, Arkema cautioned. In some instances poor and socially vulnerable people stand to bear the brunt of the disaster, representing a high human cost even with lower economic consequences.

The model study covered the entire U.S. coast, and so by necessity used data on a national scale that sometimes lacked detailed local information on exact habitat locations and conditions. "Where The Nature Conservancy is involved is the next step," said Kareiva.

"We can take this study and go to Florida or the Gulf of Mexico, North Carolina, and New York and say these are the areas where natural habitat can really reduce risk," he said. "Now we have to actually map the habitat that's there and be more precise to know exactly where it is."

Putting this into practice, The Nature Conservancy is using this same model on a localized scale in the Gulf of Mexico, combined with field experiments, to evaluate where to restore oyster reefs, how to design them, and what sizes to make them, Kareiva explained.

"FEMA spends half a billion dollars a year on risk mitigation for floods, in response to local communities. We're working with local communities to say hey all this money doesn't need to go into concrete," he added.

"Certainly we're always going to need levees but some of this money could be going into habitat protection and that turns out to also be good for fisheries, recreation, and lots of other areas."

Insurers Take Note, But Habitat Help Is Hard to Value

Communities around the world face similar challenges, and enjoy similar opportunities for natural protection.

The Nature Conservancy, the German Alliance for Development Works, and the United Nations University Institute for Environment and Human Security (UNU-EHS) led the World Risk Report, which focused on the role of the environment in reducing risk of natural disasters, and how risks rise with environmental degradation.

With so much already at stake, and coastal development continuing apace, the insurance industry is also taking note of the role played by natural habitats.

A 2011 insurance industry report, backed by 16 Caribbean governments, recommended that restoring reefs and mangroves are among the most cost effective ways to protect people and property from natural disasters in the region. A study sponsored by America's Energy Coast, America's Wetlands Foundation, and power producers Entergy Corporation looks at similar issues along the U.S. Gulf Coast.

Swiss Re—a reinsurer, or backup insurer for insurance companies—contributed to both of these reports. "What we've tried to do with our Economics of Climate Adaptation work is analyze the value of the natural infrastructure from the point of view of how it could reduce the expected losses from a major weather event," said Mark Way, Head of Sustainability Americas at Swiss Re.

"On the Gulf Coast, for example, we looked at what it would cost to do wetlands restoration and what the possible benefits would be," he said.

But Way said that this type of financial analysis remains difficult at the present time.

"It's hard to have a comprehensive evaluation of the true benefits of natural capital in the sense of the auxiliary benefits you get," he said. "You can create buffers against the weather, but at the same time you benefit leisure activities, fisheries, the local environment. We haven't really quantified those yet."

Way's colleague at Swiss Re, Andy Castaldi, added that while the benefits of natural-habitat protection are real to most of the insurance industry, analysis of them is not yet greatly impacting the bottom line.

"To most of the market I think it's not yet tangible because no one is helping them to understand and assess habitat improvements," said Castaldi, who is head of Swiss Re's Catastrophe Perils, Americas division.

"So right now, while I think most every insurance company would support protecting a barrier island or mangrove forest, at this time there is no way for them to factor this benefit into their underwriting judgment. No one has quantified it for them on the entire coastline basis."

But if the total costs and benefits involved are hard to quantify, protection of the key coastal habitat areas in the study will not only keep people safe but ultimately save money as well, study leader Katie Arkema stressed.

"It's going to cost us a lot more to try to engineer solutions like seawalls, or to try to restore habitats once we've lost them," she said.

"It doesn't take as much investment to conserve them while we've got them. So our research suggests, let's retain them—especially where we've got a lot of people. We can be creative, and mix these natural protections with hard engineering as well, but we certainly don't want to lose the protection we're receiving from the ecosystems we have intact."